An electronic circuit that produces an output frequency which is an integral multiple of the input frequency is known as a frequency multiplier
One type of frequency multiplier uses the nonlinear capacitance of a semiconductor junction diode to couple energy from an input circuit which is tuned to a fundamental input frequency to an output circuit which is tuned to a desired harmonic of the input frequency. A semiconductor junction device which has a nonlinear capacitance is known as a varactor. The basic physics of the varactor is described in S. M. Sze, Physics of Semiconductor Devices, 2nd Edition, Wiley 1981, pp. 114-116. Such varactor based frequency multiplier circuits are widely used in parametric amplification, mixing, detection, and voltage variable tuning. Varactor based frequency multiplier circuits are disclosed in Penfield and Rafuse "Varactor Applications" published by MIT Press in 1962.
One drawback to varactor based frequency multiplier circuits is that it is difficult to use these circuits to generate higher order harmonics of the input frequency. The reason for this is that the varactor has a capacitance which is weakly nonlinear. When a weakly nonlinear device such as a varactor is driven with an input frequency .omega..sub.o, the predominant output is the harmonic frequency 2.omega..sub.o. In order to generate higher order harmonics such as 3.omega..sub.o, 4.omega..sub.o etc., complex circuit arrangements are needed to feed the 2.omega..sub.o output back to the input. In order to generate the 5.omega..sub.o, 6.omega..sub.o, and 7.omega..sub.o harmonics, the 3.omega..sub.o and 4.omega..sub.o harmonics must be fed back to the input. In general, the required feedback circuitry becomes more and more complicated as one goes to higher and higher harmonics of the input frequency. In addition, the complex feedback circuitry makes it difficult to change the multiple of the input frequency produced by the frequency multiplier circuit, as each individual multiple of the input frequency requires a different feedback circuit arrangement.
Accordingly, it is an object of the present invention to provide a frequency multiplier circuit which produces higher order harmonics of an input frequency without the use of complex feedback circuits. It is a further object of the present invention to provide a frequency multiplier circuit in which it is possible to adjust in a simple manner the multiple of the input frequency produced by the frequency multipler circuit.